Conventionally, in an imaging device such as a digital camera, with advance in a high performance of a lens barrel having a zoom lens function capable of changing a focal distance, or miniaturization of the imaging device according to the user's demand, there has been increasingly used a collapsible lens barrel in which a plurality of lens groups are collapsed when is not in photograph. Furthermore, because of the demand for not only simple reduction in dimension, but also further reduction in thickness, it is now important to reduce the thickness of the lens barrel in a collapsed position (dimension in a direction of an optical axis) to the maximum position.
As a technology to cope with the demand for reduction in thickness of the lens barrel, there has been used a collapsible lens barrel in which a lens cylinder retaining therein a plurality of lens groups is collapsed into the imaging device when not in use, and one of the lens groups is retracted out of an optical axis of the lens groups when the lens groups are collapsed.
A technology for retracting one of lens groups out of an optical axis is disclosed in, for example, JP No. 2003-315861 A (Patent Document 1) and JP No. 2003-149723 A (Patent Document 2). According to the structures disclosed in these Patent Documents 1 and 2, since one of a plurality of lens groups disposed on the optical axis is retracted out of the optical axis when the lens groups are collapsed, the entire dimension of the lens in a direction of the optical axis can be shortened by a length of the retracted lens group in a direction of the optical axis and the thickness of the imaging device can be reduced.
However, in the structures disclosed in Patent Documents 1 and 2, the lens retracted out of the optical axis displaces merely within a collapsible lens cylinder (telescopic cylinder) provided in the lens barrel, therefore even the thickness of the imaging device can be reduced when the lens groups are collapsed, a greater outer diameter of the lens cylinder must be secured compare with a camera in which there is no lens retracted out of the optical axis. As a result, the size of the lens cylinder, especially the dimension in a plane transverse to the optical axis has to be increased, eventually results in a larger size of the imaging device especially viewed from the front.
To solve the problems, technologies about a lens barrel capable of retracting one of the lenses out of a telescopic cylinder is proposed (Document 3, unpublished Japanese patent application JP No. 2005-044909). According to the technology, the lens retracted from an optical axis is not collapsed inside of the telescopic cylinder at the position with maximum outer diameter, so that the maximum outer diameter of the telescopic cylinder is not required to be greater and therefore a compact outer diameter of the telescopic cylinder can be obtained.
However, when the optical system comprising lens groups disposed in a lens barrel is used as zooming optical system, a moved amount of the lens is required to be increased to obtain a greater changing magnification ratio.
In addition, a longer distance from a front side of the lens cylinder (subject side) to a rear end side (imaging side) of a fixed base member retaining the lens cylinder should be secured to obtain a greater driven amount for the retracted lens group.